Dial-up modems, also known as analog modems, analog dial-up modems, or analog data modems, are common equipment found in or connected to many computers today. One of the most common types of dial-up modems are analog soft-modems, also referred to as soft-modems. Soft-modems are simple devices comprising a minimal amount of circuitry such as codecs for converting analog signals to digital signals and converting digital signals to analog signals, circuitry for interfacing the soft-modem with a microprocessor in a host computer, and circuitry for interfacing the soft-modem with a telephone line of the public switched telephone network (PSTN). The processing of communication data and running of communication protocols is accomplished by software running on the processor of the host computer, thus the term “soft” modem.
Soft-modems run ITU-T V series standard protocols, such as the V.90 standard, on the host computer's processor, but may run an array of protocols by changing the software, or modem driver, executing on the processor. This makes soft-modems upgradeable as new and better communication protocols are developed. It also makes soft-modems inexpensive since the normally expensive circuitry needed to run communication protocols is absent from the soft modem.
Dial-up modems connect to regular voice telephone lines and dial the telephone network the same way a person dials-up another person. The modem takes the telephone line off-hook by a applying a current load to the line of about 20 mA. The current load signals a central office switch that a user needs service. The switch then applies a dial-tone to the telephone line. When the modem hears the dial-tone, the modem dials an Internet Service Provider (ISP) using standard DTMF (dual tone multi-frequency) tones. The switch then connects the customer premise equipment (CPE), in this case the dial-up modem, with a Central Office (CO) modem. The CO modem and the CPE modem then negotiate a connection, or communication link, using audible tones according to a standard such as the ITU-T V.8 standard.
Signal-to-noise (SNR) ratio and bandwidth both impact a modem's ability to effectively transfer information over a communication link. Shannon's law, expressed in the form C=BW*Log2(1+SNR), defines the theoretical maximum rate at which error-free digits can be transmitted over a bandwidth-limited channel in the presence of noise. C is the channel capacity in bits per second (bps), BW is the bandwidth of the bearer channel in Hz, and SNR is the signal to noise ratio of the bearer channel.
The PSTN terminates calls, such as the negotiated communication link described above, with an 8 bit 8 kHz codec. This limits the bandwidth of the bearer channel, BW, to approximately 4 kHz. In practice, the spectrum from around 200 Hz to 3.4 kHz is used further limiting the bandwidth of the bearer channel to around 3.2 kHz. Additionally, output transmission levels on the PSTN are limited to −9 dBm. Therefore, the SNR varies from loop to loop (the telephone lines connecting the customer premise equipment with the terminating equipment) according to loop length, bundle make up, and other PSTN and system characteristics. While noise can vary, the noise floor for a loop is generally accepted to be approximately −41 dB and the SNR is generally accepted to be 32 dB. Thus according to Shannon's law, the theoretical maximum capacity C is approximately 34 kbps. Turbo Coded Modulation (TCM), as implemented in the ITU-T V.34 standard, describes the methods used to approach the theoretical maximum limit.
By taking advantage of the CO side circuitry operating at 8 kHz, the 3.4 kHz limit imposed by the PSTN is doubled and the bandwidth of the bearer channel BW is increased by a factor of two to achieve a theoretical maximum capacity C of approximately 64 kbps. The ITU-T V.90 standard describes the methods used to achieve communications close to the theoretical maximum limit. V.90 is currently the fastest standard for analog dial-up modem communications.
Digital subscriber line (DSL or xDSL) modems, which communicate using a variety of DSL standards such as ADSL, VDSL, SHDSL, SDSL and HDSL, do not dial through the PSTN as described above. Rather, the CO and the CPE modems are directly wired to each other. This direct connection allows for usable bandwidth of up to around 12 MHz depending on the length of the loop. The bandwidth is often shared with the PSTN, for example during ADSL and VDSL communications, thereby allowing voice calls and data calls to coexist on the same line.
While DSL communications are superior in terms of speed over their analog counterparts discussed above, many computer users do not have DSL modems and are therefore limited to the comparatively slow speeds of their analog dial-up modems. Therefore, a need presently exists for a faster modem method and apparatus.